The complement system is proteolytically modulated by the C3bINA-C4bINA system which is generally believed to cleave C3b into two fragments, C3c and C3d. However, little is known as to the exact mechanism of the proteolytic action of C3bINA on C3b and C4b. Recently, we documented the requirement of a macromolecular weight co-factor for the proteolytic function of C3bINA (Immunochemistry, in press). On the other hand, it was recently reported that β1H plays a role also as a co-factor for the proteolytic activity of C3bINA.

In the present study, we have purified the macromolecular weight co-factor and characterized it as a new serum protein. Also, we have highly purified the C3bINA by affinity chromatography with C3b-Sepharose, in order to study the cleavage mechanisms.

Purification and characterization of the macromolecular weight co-factor.

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